This invention relates to the process control of a flue gas desulfurization system that utilizes a sodium sulfite- and sodium bisulfite-containing alkaline absorption solution regenerated with limestone.
Flue gas desulfurization processes that employ alkaline absorption solutions containing both sodium sulfite and sodium bisulfite for removing sulfur oxides like SO.sub.2 from flue gases are well known and are normally operated continuously with the spent absorption solution being regenerated with lime or limestone. Such lime/limestone-sodium salt processes are often termed "double" or "dual" alkali processes in contradistinction to single alkali processes that directly treat the flue gas with lime or limestone.
Double alkali processes that utilize lime as the regenerating agent are in commercial use throughout the United States. U.S. Pat. No. 3,911,084 issued to Wall et al. discloses one such lime-sodium double alkali process.
Double alkali processes that utilize limestone, however, are not known to be in commercial use in the United States, despite the cost advantages of limestone over lime. This is apparently so because no efficient procedures, i.e., parameter ranges and process control techniques, have been devised for operating such processes competitively with lime-sodium double alkali processes.
Lime-sodium double alkali processes described in the patent literature often include the disclosure that limestone may be substituted as the regenerating agent for lime. Such a substitution, however, is not at all straightforward, particularly with respect to process control, since regeneration characteristics such as regenerating agent reaction rate and utilization efficiency, and solution pH values, are distinctly different for these two chemical regenerating agents.
In a lime-based regeneration system, lime addition is satisfactorily controlled via a feedback control signal from regeneration pH, to provide the desired stoichiometric quantity of lime for completely neutralizing the absorbed sulfur oxides. The regeneration pH is very responsive and sensitive to the lime addition rate, because of lime's high reactivity with bisulfite formed from absorbed SO.sub.2, the short reactor residence times typically employed, and the very alkaline pH (usually in excess of 8.5) required for complete regeneration.
Unlike lime, limestone is characterized by low reactivity with bisulfite, requiring fairly long regeneration reactor residence times. This factor, along with the relatively low regeneration pH values employed in limestone regeneration and the highly buffered nature of a limestone-sodium absorber solution, preclude the use of regeneration pH as the primary parameter for satisfactorily controlling the limestone addition rate during regeneration.
Sodium-limestone double alkali systems that are described in the prior art, such as U.S. Pat. No. 3,848,070 issued to Onozuka et al., No. 3,944,649 issued to Field et al. and No. 3,989,796 issued to Morita et al., provide little information concerning effective and efficient process control of the absorption and regeneration processes.
The present invention provides a method for controlling the continuous operation of a sodium-limestone double alkali flue gas desulfurization system with superior efficiency in both the absorption and regeneration processes. The method is particularly suited for controlling the sodium-limestone double alkali flue gas desulfurization method described in co-pending application U.S. Ser. No. 376,879 filed May 10, 1982, whose disclosures are hereby incorporated by reference.